In RF circuits combining a frequency synthesizer and a QAM modulator (quadrature amplitude modulation), leakages of fundamental, harmonic or mixed signals from the QAM modulator deteriorate the performances of the voltage-controlled oscillator present in the frequency synthesizer, due to an injection pulling effect also known as injection locking.
FIG. 1 represents a radiotelephony RF integrated circuit in which the injection locking effect constitutes a fundamental problem for those skilled in the art. This RF integrated circuit comprises a frequency synthesizer 10 and a QAM modulator, the QAM modulator here forming a transmit radio frequency circuit TXCT.
The frequency synthesizer 10 is driven by a quartz oscillator 20 and supplies to the circuit TXCT an output signal of frequency F1 adjustable with a determined frequency step. Typically, the frequency F1 is of 3.6 GHz and is adjustable from 3.2 GHz to 4 GHz with a step of 400 KHz.
In the following description, the “rank” of a frequency divider will designate the division coefficient of the frequency divider considered.
The frequency synthesizer 10 comprises a voltage-controlled oscillator 11 (VCO), a frequency divider 12 of rank R and a phase-locked loop. The phase-locked loop comprises a frequency divider 13 of rank N, a phase comparator 14 (PFD) and a loop filter 15 of low-pass type. The rank N of the frequency divider 13 is programmable by means of a signal CS (“Channel Selection”) applied to a control input of the frequency divider.
The output signal of frequency F1 is supplied by the oscillator 11, and is sent back to the input of the frequency divider 13. The latter supplies a signal of frequency F1/N that is applied to a first input of the phase comparator 14. The phase comparator receives at a second input a comparison signal of frequency Fcomp, supplied by the frequency divider 12 the input of which receives a signal of frequency Fq supplied by the quartz oscillator 20 (Fcomp=Fq/R). The output of the phase comparator 14 supplies a control signal Vcont that is applied to the oscillator 11 through the loop filter 15. The signal of frequency F1 is thus frequency and phase controlled, and the frequency F1 is equal to N*Fcomp, Fcomp representing the frequency step of the output signal. Typically Fcomp is in the order of 400 KHz for a baseband frequency FBB of 200 KHz, and N varies between 8,000 and 10,000.
The circuit TXCT or QAM modulator receives an analog signal Sx and the signal of frequency F1, and supplies an antenna signal RFSx that is phase modulated. The signal RFSx is applied to an RF antenna that is not represented on the Figure.
The signal of frequency F1 is applied to a frequency divider DIVK of rank K, the output of which supplies a carrier Fc of the radio channel, Fc being equal to F1/K, K generally being equal to 2 for the DCS band (1.8 GHz) and to 4 for the GSM band (900 MHz).
The signal Sx is digitised by a converter ADC, then is applied to an encoder modem CODEM then is again applied to a processor IQGEN. The processor IQGEN supplies, in the baseband of frequency FBB, a phase signal I and a quadrature signal Q to be modulated with the carrier Fc. The signal I is applied to an input of a mixer IMIX through an amplifier IAMP, and the signal Q is applied to an input of a mixer QMIX through an amplifier QAMP. The mixer IMIX receives the carrier Fc at another input and the mixer QMIX receives at another input the carrier Fc off-phased by 90°, supplied by a phase-shifter DPH. The outputs of the mixers IMIX, QMIX are applied to an adder IQAD that supplies the modulated signal RFSx. The signal RFSx is applied to an output amplifier RFAMP the output of which supplies the antenna signal.
The signal Sx generally contains data to be transmitted, such as a coded voice for example, and has a frequency spectrum representative of the modulation scheme provided for by the standard implemented (for example GMSK in GSM). By considering as an example that the signal Sx is a single tone, the circuit IQGEN then supplies two pure quadrature sinusoids I=cos(FBB) and Q=sin(FBB). The result of the phase modulation IQ is in this case a single tone of frequency Fc+FBB the image component Fc−FBB of which is in principle removed by the quadrature modulation, and the carrier Fc of which is also removed, or at least greatly attenuated.
Due to imperfections in the modulation circuit and in the output amplifier RFAMP, or non-linearities, the signal RFSx comprises, in addition to the useful component H1 of frequency Fc+FBB, harmonic components H2, H3, H4, . . . Out of these components, at least one is close to the oscillation frequency F1 of the oscillator 11 of the frequency synthesizer 10 and forms a spurious component that is re-injected into the oscillator via various spurious paths (electromagnetic coupling and propagation in the substrate). In particular, it is the second harmonic H2 when the frequency divider DIVK is a divider by 2 (DCS band) or the fourth harmonic H4 when the frequency divider DIVK is a divider by 4 (GSM band). Indeed, when K=2 the frequency of the second harmonic H2 is equal to 2Fc+2FBB (i.e., F1+2FBB) and is very close to the central frequency F1 of the oscillator 11 since the baseband frequency FBB is low in relation to the carrier Fc. Similarly, when K=4 the fourth harmonic H4 has a frequency of 4Fc+4FBB (i.e., F1+4FBB) that is close to the central frequency of the oscillator 11. This spurious component can also be the useful component H1 itself, in a circuit in which K is equal to 1.
It is well known that the involuntary injection of this spurious component into the core of the oscillator deteriorates the performances of the latter. Indeed, the phase-locked loop does not manage to totally remove the spurious component that is therefore re-injected into the output signal of the voltage-controlled oscillator. An injection locking effect follows, since the spurious component re-injected into the output signal of the oscillator then comes back into the core of the oscillator via spurious paths, is again re-injected into the output signal, and so on and so forth.
Various solutions are known to overcome this disadvantage.
First of all, it is frequent for the voltage-controlled oscillator to be produced on a silicon microchip distinct from the one bearing the circuit TXCT. However, this solution is complex to implement and burdens the cost price of the RF circuits, which is passed on, at the end of the line, to the selling price of mobile telephones. Thus, the current tendency is, on the contrary, to integrate the oscillator into the silicon microchip bearing the transmission circuit TXCT. Integrating, onto the same silicon microchip, the output amplifier RFAMP that is not linear and constitutes a considerable source of high-amplitude spurious harmonics, is also considered.
Another solution involves shifting the frequency of the carrier Fc in relation to the central frequency of the voltage-controlled oscillator. Thus, the heterodyne systems use several voltage-controlled oscillators and several cascaded mixers, and a pre-modulation stage using an intermediary frequency IF. In the output stage, the carrier frequency of the modulated signal is shifted relative to the natural frequency of the voltage-controlled oscillator, and the signals likely to interfere with the voltage-controlled oscillator are harmonic and/or high-rank mixed products that are greatly attenuated. Heterodyne systems are however complex and costly to produce, as are the additional mixers and filters.
Yet another solution involves providing a frequency synthesizer supplying an output signal the frequency of which is shifted relative to the central frequency of the voltage-controlled oscillator.
FIG. 2 represents a circuit conforming to this solution. The circuit represented comprises two frequency synthesizers 10, 10′ driven by the same quartz oscillator 20. Each frequency synthesizer is of a structure similar to the one in FIG. 1 and comprises a voltage-controlled oscillator 11, 11′, a frequency divider 12, 12′ of rank R, a frequency divider 13, 13′ of rank N, a phase comparator 14, 14′ and a loop filter 15, 15′. Each voltage-controlled oscillator 11, 11′ supplies a signal of frequency F1, respectively F2. The two signals are applied to a mixer OUTMIX which supplies an output signal comprising an additive component of frequency F1+F2 and a subtractive component of frequency F2−F1. The output signal of the mixer is applied to a filter OUTFLT that removes one of the components, such as the subtractive component for example, so as to keep only the other component as output signal of the circuit. Thus, the harmonic signals of the output signal, of frequency F1+F2, are far from the central frequencies of each of the voltage-controlled oscillators, and their interference is reduced.
Along the same lines, FIG. 3 represents a frequency synthesizer 10″ as described by U.S. Pat. No. 6,321,1074. The frequency synthesizer is driven by a quartz oscillator 20 and comprises, like the one in FIG. 1, a voltage-controlled oscillator 11, a frequency divider 12, a frequency divider 13, a phase comparator 14 and a loop filter 15. A frequency divider 16 of rank 2 is here inserted into the phase-locked loop, between the output of the oscillator 11 and the input of the frequency divider 13. The output signal of the oscillator 11, of frequency F1, is applied to an input of an output mixer OUTMIX another input of which receives the signal supplied by the frequency divider 16, of frequency F1/2. The output signal of the mixer thus comprises an additive component of frequency F1+F1/2 i.e., 3/2*F1 and a subtractive component of frequency F1−F1/2 i.e., F1/2. An output filter OUTFLT removes the subtractive component. Thus, the harmonics of the output signal have frequencies (3*F1, 4,5*F1, 6*F1 . . . ) that are far from the central frequency F1 of the oscillator 11, and their interference is reduced.
The disadvantage of these two frequency synthesizers is that the output signal is supplied by a mixer, and that it is necessary to greatly filter the output of the mixer so as to only keep the useful frequency. Such filtering most often requires an external component, and increases the cost price and the complexity of the RF integrated circuit.